runtime/internal/atomic: export more ABI0 wrappers

Somehow I missed these two functions in CL 179863. This should fix the
linux-arm builders.

Updates #31230.

Change-Id: I3f8bef3fac331b505a55c0850b0fbc799b7c06c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/181077
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/internal/atomic/atomic_arm.go

@@ -11,6 +11,10 @@ import (
 	"unsafe"
 )
 
+// Export some functions via linkname to assembly in sync/atomic.
+//go:linkname Xchg
+//go:linkname Xchguintptr
+
 type spinlock struct {
 	v uint32
 }